Recordable or rewritable optical disks have now become popular. CD-Rs (Compact Disc Recordable) and CD-RWs (Compact Disc Rewritable) are typical recordable or rewritable optical disks. In recent years, DVD-Rs (Digital Versatile Disk) and DVD-RWs (DVD ReWritable), which have recording capacities larger than CD-Rs and CD-RWs, are commercially available.
Recording data onto recordable optical disks is often achieved by phase change recording. Phase change recording involves partially changing a recording layer made of phase change recording material in quality through irradiating a laser beam, and thereby forming recording pits thereon. Irradiating a laser beam increases the temperature of the irradiated portions. The temperature increase changes the irradiated portions in quality, and thereby alters the reflectance of the irradiated portions. The irradiated portions, with their reflectances altered, are used as recording pits. The reflectances of the irradiated portions may be decreased or increased.
Phase change recording designed to decrease the reflectance of recording pits is preferable from the viewpoint of the compatibility between recordable optical disks and read-only optical disks (namely, CD-ROMs). CD-ROMs is designed to record data thereon by using pre-pits, whose reflectances are reduced by embossing. Recording pits on recordable optical disks, which have reduced reflectances, exhibit the same effect as the pre-pits on CD-ROMs. Therefore, recordable optical disks can be designed so as to have the compatibility with the read only optical disks (namely, CD-ROMs).
Phase change recording designed to increase the reflectance of recording pits, on the other hand, is advantageous for improving an S/N ratio of an optical signal reflected by optical disks. In most recordable optical disks, the size of the region where recording pits are formed is larger than that of a space (namely, the portions except the region where recording pits are formed) Optical disks that have the decreased reflectance in the space and the increased reflectance in the recording pits reduce the average of levels of reflected light beams, and thereby effectively improve the S/N ratio. The improvement in the S/N ratio is commercially desirable because the improvement of the S/N ratio allows the improvement of the recording density, namely, the increase in the amount of recordable data for a single optical disk.
FIG. 1 is a plan view of an optical disk 101 for phase change recording designed to increase the reflectance of recording pits. A spiral recording track 104 is formed on the optical disk 101. The recording track 104 is divided into sectors each having a predetermined sector length. Each sector contains header information (or format information) including the sector address at the head portion. The portions where the header information is recorded are referred to as header regions 105. User data are recorded in the remaining portions of the sectors. The portions where the user data are recorded are referred to as data recording regions 106.
FIG. 2 shows the cross sectional structure of the optical disk 101. The optical disk 101 includes a transparent substrate 103 and a recording layer 102 covering the transparent substrate 103. The recording layer 102 is made of phase change recording material which has a reduced reflectance in the crystalline phase compared to that in the amorphous phase.
The data recording regions 106 are provided with recording pits 111 corresponding to the user data in the recording layer 102. A light beam used for data reproduction is transmitted across the transparent substrate 103 to reach the recording layer 102, and is reflected by the recording layer 102. A solid line in FIG. 1 indicates the reflection of the light beam by a recording pit 111, and a broken line indicates the reflection of the light beam by a space 112 of the recording layer 102.
The recording pit 111 is the portion exhibiting the amorphous phase within the recording layer 102, and the space 112 is the portion exhibiting the crystalline phase. This configuration makes the reflectance in the recording bit 111 higher than the reflectance in the space 112.
Such design of the recording layer 102 effectively improves the S/N ratio. The crystalline portion of the recording layer 102 exhibits minute non-uniformity over the area, because the crystalline portion of the recording layer 102 is formed of a group of micro crystals. The minute non-uniformity in the reflectance causes noise. The amorphous portion, on the other hand, exhibits reduced noise. The configuration in which the reflectance of the amorphous portion is high and the reflectance of the crystalline portion is low reduces the level of the light reflected from the portion that causes large noise, and thereby improves the S/N ratio.
The header regions 105 are provided with pre-pits corresponding to the header information. Embossed patterns are formed inside the pre-pits. The embossed patterns reduce the reflectance of the pre-pits below that of the remaining portions of the header regions 105.
One problem of such phase change recording is the reliability in reading out the header information. The optical disk 101 for the phase change recording designed to increase the reflectance of the recording pits have the header region 105 formed within the crystalline portion of the recording layer 102 (namely, the portion where the reflectance is low). The decrease in the reflectance of the header region 105 further decreases the reflectance of the pre-pits formed in the header region 105. This may reduce the level of the optical signal reflected from the header region 105, and reduce the reliability in reading out the header information. FIG. 3 is an example of an eye-pattern of reproduced signals obtained from the optical signals reflected from the header region 105 and the data recording region 106. The reproduced signals are obtained by converting the optical signals into electric signals, cutting the direct current components of the electric signals, and extracting only the alternating current components. As indicated by the eye-pattern, the amplitude of the level of the reproduced signal obtained from the header region 105 is smaller than that of the reproduced signal obtained from the data recording region 106. This deteriorates the reliability in reading the header information written in the header region 105.
The header information (or format information) can be recorded in a wobble groove for tracking, instead of the pre-pits. Recording header information onto the wobble groove is achieved by modulating the wobble pattern of the wobble groove in response to the header information. This method is referred to as a wobble modulation and applied to CD-Rs. However, the wobble modulation intrinsically exhibits a reduced S/N ratio since the amplitude of the wobble pattern is limited. Moreover, the reduction in the S/N ratio is severe for the phase change recording designed to increase the reflectance of recording pits, because this requires forming the wobble groove in the crystalline portion (namely, the portion where the reflectance is low).
A technique for improving the reliability in reading out the header information is disclosed in Japanese Laid Open Patent Application (JP-A No. P2000-311343A). This document discloses a technique of increasing the reproduction amplitude of an address signal (namely, header information) by controlling a disk drive so as to make the level of the light beam used for the reproduction of an address region (namely, a header region) stronger than that of a data region (namely, a data recording region).
Additionally, a technique of improving the degree of the modulation of pre-pit portions is disclosed in Japanese Laid Open Patent Application (JP-A No. P2000-339690A). This document discloses a technique that improves the modulation degree through irradiating a laser beam on pre-pit portions within the recording layer during manufacture so that the pre-pit portions are changed into the recorded state.